Prostate cancer is the second leading cause of cancer death among men in the United States (US), with ~30,000 deaths expected in 2014. Most morbidity and nearly all mortality due to prostate cancer occur once the disease metastasizes to bone and becomes castration-resistant (mCRPC); the expected survival of men with mCRPC is 2-4 years. Identifying low-toxicity interventions that improve outcomes in this population is a high priority and has the potential to have a large impact on the clinical and public health burden of prostate cancer. Exercise may be an ideal non-pharmacologic therapy for men with mCRPC; many men with mCRPC are able to be physically active, and exercise may minimize treatment-related morbidity and disease progression. Men with mCRPC are generally excluded from exercise trials due to safety concerns, but supervised resistance exercise was recently reported to be safe and improve quality-of-life among 20 Australian men with metastatic prostate cancer. It remains unknown whether supervised aerobic exercise is safe and feasible or whether aerobic or resistance exercise affect prognosis in men with mCRPC. Furthermore, no prior study has compared the safety and efficacy of aerobic vs. resistance exercise in mCRPC. With a long- term goal of conducting a definitive randomized controlled trial evaluating the efficacy of exercise to minimize morbidity and delay death among men with mCRPC, we propose a pilot 3-arm (aerobic, resistance, usual care) 12-week randomized controlled trial among 75 men with mCRPC. Both the aerobic and resistance exercise programs minimize compressive and shear loads on sites of metastases, and consist of 35-60 minutes of exercise three days per week under supervision of an exercise physiologist. The resistance arm will perform up to 3 sets of 6-10 reps of exercises targeting major trunk, upper body, and lower body muscle groups; the aerobic arm will perform treadmill walking or stationary cycling at 60-85% estimated maximal heart rate; and the control arm will receive standard-of-care. All men will receive educational materials at baseline. Our aims are to investigate whether supervised aerobic or resistance exercise is safe and feasible among men with mCRPC; whether these modes of exercise improve a validated prognostic score (based on circulating biomarkers and clinical factors) compared to usual care; whether supervised aerobic or resistance exercise impact fatigue, physical function, and quality-of-life compared to usual care; and whether resistance exercise has a greater effect on the prognostic score and quality-of-life than aerobic exercise among men with mCRPC. The results of this pilot randomized controlled trial will directly inform the design of a large, definitive trial examining the effect f exercise on disease-free and overall survival among men with mCRPC. Such a trial will facilitate the development and implementation of exercise programs for men with mCRPC, directly inform clinical guidelines, and improve the clinical management of metastatic prostate cancer.